Highly unsaturated fatty acid is a collective term of fatty acids having many double bonds. Attention has been paid to the biological functions of highly unsaturated fatty acids and derivatives thereof, especially icosapentaenoic acid and docosapentaenoic acid derived from fish oils. The demand thereof has been increasing, including high-purity icosapentaenoic acid as a pharmaceutical. Thus, there is a demand for a novel production method of a highly unsaturated fatty acid ethyl ester, or novel purification method of a highly unsaturated fatty acid ethyl ester, that is more simple and economic, and that could achieve higher purity and/or higher yield.
A silver complex method, which utilizes the characteristic of highly unsaturated fatty acids and derivatives thereof that they forma complex with silver ions and become water-soluble, is known as one of purification methods of highly unsaturated fatty acids (Patent Literatures 1-4). Patent Literatures 1-4 describe that silver salt that is used for purification of highly unsaturated fatty acids and derivatives thereof can be reutilized. However, silver salt has a property of being extremely liable to deterioration. When highly unsaturated fatty acids and derivatives thereof are purified by using deteriorated silver salt, contamination of impurities and deterioration of flavor would occur, and it is not possible to obtain good purified products. Accordingly, reutilization of silver salt is extremely difficult in reality, and when purifying highly unsaturated fatty acids and derivatives thereof on an industrial scale, there is a problem that a new silver salt aqueous solution needs to be prepared each time, and the purification cost becomes extremely expensive.
In order to provide highly unsaturated fatty acids and derivatives thereof having a good quality level at a low cost, a method of allowing the free fatty acid content in a silver salt aqueous solution that will be reutilized to be below a predetermined value, was developed as a technique of enabling repeated reutilization of a silver salt aqueous solution for a long period of time (Patent Literature 5). This method is characterized in that a silver salt aqueous solution is made into contact with a free fatty acid reducing agent to manage the free fatty acid content in the silver salt aqueous solution to be 0.2 meq or smaller per 1 g of silver. However, as was made clear by the inventors and as will be described below, if only the acid value of raw materials is focused and attention is not paid to the peroxide value, the peroxides of the raw materials may cause silver nitrate deterioration in the silver nitrate aqueous solution, and may cause decrease in production yield and the like. As will be shown below, the inventors made clear that peroxides in raw materials cause silver nitrate deterioration in silver nitrate aqueous solutions and cause decrease in production yield and the like, and the inventors completed the present invention characterized in that the peroxide value in raw materials is defined to 10 or smaller to decrease silver nitrate deterioration. Although Patent Literature 5 describes an acid value regarding the quality of raw materials, it does not have descriptions concerning a peroxide value. An acid value, which is the quality index of raw materials, does not correlate with a peroxide value. Thus, the subject matter described in Patent Literature 5 may develop a problem that silver nitrate deterioration in a silver nitrate aqueous solution is caused by the peroxides of raw materials, and decrease in production yield and the like are caused.